The COP9 signalosome and its role in plant development.

The COP9 signalosome (CSN) is an evolutionarily conserved multiprotein complex with a role in the regulation of cullin-RING type E3 ubiquitin ligases (CRLs). CSN exerts its function on E3 ligases by deconjugating the ubiquitin-related protein NEDD8 from the CRL cullin subunit. Thereby, CSN has an impact on multiple CRL-dependent processes. In recent years, advances have been made in understanding the structural organisation and biochemical function of CSN: Crystal structure analysis and mass spectrometry-assisted studies have come up with first models of the pair-wise and complex interactions of the 8 CSN subunits. Based on the analysis of mutant phenotypes, it can now be taken as an accepted fact that--at least in plants--the major biochemical function of CSN resides in its deneddylation activity, which is mediated by CSN subunit 5 (CSN5). Furthermore, it could be demonstrated that CSN function and deneddylation are required but not essential for CRL-mediated processes, and models for the role of neddylation and deneddylation in controlling CRL activity are emerging. Significant advances have also been made in identifying pathways that are growth restricting in the Arabidopsis csn mutants. Recently it has been shown that a G2 phase arrest, possibly due to genomic instability, restricts growth in Arabidopsis csn mutants. This review provides an update on recent advances in understanding CSN structure and function and summarises the current knowledge on its role in plant development and cell cycle progression.